1. Field of the Invention
The present invention relates to a reflection type liquid crystal display device for displaying by reflecting the incident light from the outside, which is preferably used as display means of portable information terminal devices, portable word processors, personal computers and so on.
2. Description of the Related Art
Conventionally, liquid crystal display devices are widely employed as display means of information terminal devices such as personal computers, word processors, and electronic pocket notebooks, portable type television apparatuses, and the like. In the case of a liquid crystal display device of black-and-white display image, a TN (twisted nematic)-type liquid crystal display device is used for an electronic apparatus such as an electronic pocket calculator, and an electronic watch in which the amount of information to be displayed is relatively small, and a STN(super twisted nematic)-type liquid crystal device is used for an electronic apparatus such as word processor in which the amount of information to be displayed is relatively large. Both TN-type and STN-type liquid crystal devices need two polarization plates, and the availability factor of the incident light from the outside is 30% or less. For that reason an image displayed in a reflection type liquid crystal display device with a reflection plate becomes relatively dark. Although a back light can be provided in order to more brighten the displayed image, that disadvantageously causes the increase of consumption power and its own weight, which is not suitable for portable electronic apparatuses.
In the ease of a color liquid crystal display device, a color filter is usually combined with either of the TN- and STN-type liquid crystal display device. Color displaying in such color liquid crystal display device is realized by an additive color process. Since two polarizing plates are used in the color liquid crystal display device the same as in a black-and-white liquid crystal display device, and accordingly the availability factor of the incident light is low. Additionally, in order to realize color displaying, picture elements are divided in to three fox red, green, and blue colors, the availability factor of the incident light is further lowered due to dividing the picture elements. Besides, in actual display panels the ratio of a region effective for display of one picture element to the picture element region depends on the light availability factor, namely, the light availability factor is lowered as the ratio of the effective region decreases. When a liquid crystal display device is directed to fine displaying, the picture element region is reduced. A region which does not contribute to displaying, for example, a region for switching devices or for wirings for supplying a voltage for displaying, however, has its limits in reducing. As a consequence, the ratio of the effective region becomes lower. For these reasons the brightness of a display image is further lowered in a color liquid crystal display device. For example, the light availability factor is lowered to a few percents. Consequently a back light is required, which is a barrier against lowering the power consumption and decreasing the weight.
In order to solve the problems, research and development for enhancing the light availability factor have been conducted. For example, it is disclosed that a display image with a high brightness and a high contrast ratio can be obtained without a polarizing plate by mixing a dichroism coloring material into the liquid crystal layer and giving a chiral structure to the orientation of liquid crystal molecules, namely, employing the White & Taylor type guest host mode (D. L. White and G. N. Taylor; J. Appl. Phys. 45 No. 114718(1974)). According to the method, the dichroism coloring material is also twist-oriented along the twist-oriented liquid crystal molecules and the light which has entered into such a liquid crystal layer is absorbed by the dichroism coloring material regardless of the polarization directions of the incident light. Thereby black color displaying can be realized. On the other hand, during the application of a voltage, the liquid crystal molecules and the dichroism coloring material are oriented along the direction of an electric field and the incident light passes through the liquid crystal layer. Thereby white color displaying can be realized.
Additionally, for example, a method wherein only one polarizing plate is used was proposed in "the 18th Liquid Crystal Discussion 3D-110(1992)". According to the method, the liquid crystal display device has the structure of polarizing plate/liquid crystal layer/reflection plate or polarizing plate/phase difference plate/liquid crystal layer/reflection plate, and displaying is carried out by phase variations of the light which has entered into the liquid crystal layer. Since only one polarizing plate is used, a relatively bright display image can be obtained.
The two above-mentioned methods make it possible to increase the light availability factor from 30% or less up to about 50%. Additionally, for example, increasing the effective region has been proposed in the Japanese Unexamined patent Publication JP-A 6-75238 (1994) of the present invention's applicant and others.
FIG. 21 is a plan view of one substrate 31 of a reflection-type liquid crystal display device 30 disclosed in said publication, and FIG. 22 is a sectional view of the reflection-type liquid crystal display device 30. A plurality of gate bus wirings 32 function of chromium, tantalum or the like are disposed in parallel with each other on the one substrate having an electrical insulating property (e.g., made of glass) and a gate electrode 33 branches off from the gate bus wirings 32. The gate bus wirings 32 function as scanning lines.
A gate insulating film 34 made of a material such as silicon nitride (SiN.sub.x) and silicon oxide (SiO.sub.x) is formed on the entire surface of the substrate 31 to envelop the gate bus wirings 32 and gate electrode 33. A semiconductor layer 35 made of, for example, amorphous silicon (hereinafter described as "a-Si"), polycrystal silicon, and cadmium selenide (CdSe) is formed on a gate insulating film 34 above the gate electrode 33. At both ends of the semiconductor layer 35 are formed contact electrodes 41 made of, for example, a-Si. A source electrode 36 made of, for example, titanium, molybdenum, and aluminium is formed to be superposed on one of the contact electrodes 41, and a drain electrode 37 made of, for example, titanium, molybdenum, and aluminium, the same as in the source electrode 36, is formed to be superposed on the other of the contact electrodes 41.
As shown in FIG. 21, a source bus wiring 39 crossing the gate bus wiring 32 via the gate insulating film 34 is connected to the source electrode 36. The gate bus wiring 39 serves as a signal line. The source bus wiring 39 is made of the same metal as that of the source electrode 36. The gate electrode 33, the gate insulating film 34, the semiconductor layer 35, the source electrode 36, and the drain electrode 37 constitutes a thin film transistor (hereinafter described as "TFT") 40, which has the function as a switching element.
An organic insulating film 42 is formed on the entire surface of the substrate 31 to envelop the source bus wiring 39 and TFT 40. A bump 42a having a height H whose top portion is formed to be spherical is formed in the region where a reflection electrode 38 of the organic insulating film 42 is formed, and a contact hole 43 is formed in a predetermined region on the drain electrode 37 of the organic insulating layer 42. In order to solve problems on the formation method of the organic insulating film 42 and on the formation process of the contact hole 43, and in order to prevent the thickness of the liquid crystal layer from becoming ununiform in manufacturing the liquid crystal display device 30, it is preferable that the height H of the bump 42a is 10 .mu.m or less. The thickness of the liquid crystal layer is generally 10 .mu.m or less. The reflection electrode 38 made of aluminium, silver, or the like is formed on the region of the organic insulating film 42 where the bump 42a is to be formed and connected to the drain electrode 47 through the contact hole 43. Additionally an orientation film 44 is formed thereon.
A color filter 46 is formed on the other substrate 45. More specifically, a magenta or green filter 46a is formed on the region opposed to the reflection electrode 38 of the substrate 31 and a black filter 46b is formed on the region not opposed thereto. A transparent electrode 47 made of, for example, ITO (Indium Tin Oxide) is formed on the entire surface of the color filter 46 and additionally an orientation film 48 is formed thereon.
The two substrates 31, 45 are laminated to be opposed to each other so that the reflection electrode 38 and filter 46 of any one of the substrates 31, 45 and those of the other of the substrates 31, 45 overlap each other, respectively, and a liquid crystal is injected between the substrates, whereby the reflection-type liquid crystal display device 30 is completed.
FIG. 23 is a plan view of a substrate 12 comprising a TFT 11 which is used for an active matrix-type liquid crystal device (Japanese Unexamined Patent Application JP-A 6-75238 (1994), and FIG. 24 is a sectional view taken on line X28--X28 of FIG. 23. A plural gate bus wirings 13 made of, for example, chromium and tantalum are disposed in parallel with each other on the substrate 12 made of, for example, glass, having an insulating property and a gate electrode 14 branches off from the gate bus wirings 13. The gate bus wirings 13 serve as scanning lines.
A gate insulating film 15 made of a material such as silicon nitride (SiN.sub.X) and silicon oxide (SiO.sub.X) is formed on the entire surface of the substrate 12 to envelop the gate electrode 14. A semiconductor layer 16 made of, for example, a-Si is formed on the gate insulating film 15 above the gate electrode 14. At both ends of the semiconductor layer 16 are formed contact layers 17 made of, for example, a-Si. A source electrode 18 is formed to be superposed on one of the contact layers 17, and a drain electrode 19 is formed to be superposed on the other of the contact layers 17. A source bus wiring 23 is formed to cross the gate bus wiring 13 via the gate insulating film 15 and connected to the source electrode 18. The source bus wiring 23 serves as a signal line. The source electrode 14, the gate insulating film 15, the semiconductor layer 16, the contact layer 17, the source electrode 18, and the drain electrode 19 constitute a TFT 11.
Additionally an organic insulating film 20 is formed thereon, which comprises plural bumps 20a and a contact hole 21 on the drain electrode 19. On the organic insulating film 20 is formed a reflection electrode 22 which is connected to the drain electrode 19 through the contact hole 21. Additionally an orientation film is formed on the reflection electrode 22. The two substrates are laminated, in the same manner as that in the substrate 31 as described above, to be opposed to each other, and a liquid crystal is injected between the substrates.
In the examples of FIGS. 21-24, since the reflection electrodes 38, 22 are formed on the organic insulating films 42, 20, the reflection electrodes 38, 22 can be partly superposed on the gate bus wirings 32, 13 and the source bus wirings 39, 23. Consequently, the areas of the reflection electrodes 38, 22 are enlarged and as a result the ratio of effective region for display is increased, which contributes to the enhancement of the light availability factor. A brighter display image can be thus obtained. Additionally, since the reflection electrodes 38, 22 made of a material having a reflection property are formed as picture electrodes and the reflection electrodes 38, 22 are used as reflection plates, the parallax is lowered in comparison with a reflection-type liquid crystal device in which a reflection plate is disposed on the opposite side of the substrates 31, 12 to the liquid crystal layer. It is possible to realize a reflection-type liquid crystal display device where a further bright display image can be obtained by the combination of a liquid crystal display device having such a constitution with the above-mentioned White & Taylor type guest-host-mode.
Although the constitution as shown in FIGS. 21-24 increases the light availability and achieves a bright display image, the following disadvantages exist due to the asperities. More specifically, the bumps 42a, 20a are formed on the surfaces of the organic insulating films 42, 20 and the reflection electrodes 38, 22 are disposed on the organic insulating films 42, 20 having bumps 42a, 20a, respectively.
A metal film to become the reflection electrodes 38, 22 is formed on each entire surface of the organic insulating films 42, 20 and patterned with a predetermined shape, whereby the reflection electrodes 38, 22 are formed. The patterning is carried out by the etching method. In etching, the metal film in an unnecessary region is dissolved and removed by an etchant. At that time, however, the etchant penetrates between the metal films to be left as the reflection electrodes 38, 22 and the organic insulating films 42, 20, respectively. The penetration of the etchant is more remarkable in the edges of the metal films to be left, the larger the interface areas of the metal films and the organic insulating films 42, 20 are. As described above, the apparent interface area becomes larger and the penetration of the etchant becomes more remarkable by forming the bumps 42a, 20a. When the organic insulation film is not firmly coated with the metal film which is formed by the spattering method, the penetration of the etchant becomes remarkable.
When the etchant penetrates between the organic insulation film and the reflection electrode, peeling of the reflection electrodes 38, 22 begins from their edges. The reflection electrodes 38, 22 peeled, the picture elements obtained are imperfect and the display quality is remarkably decreased. Additionally, since the peeled metal film of the reflection electrodes 38, 22 is found in the liquid crystal layer, there is a possibility of short circuit between the other reflection electrodes 38, 22 and the transparent electrodes opposite to the reflection electrodes 38, 22.
In the examples shown in FIGS. 21, 22, the bump 42a is not provided on the surface of the organic insulation film 42 on the wirings 32, 39, in order to prevent the short circuit between the reflection electrode 38 and the gate bus wiring 32 or the source bus wiring 39. The region where the bump 42a is not provided in order to prevent the peeling, however, is not specified.